Tadalafil is known as (6R-trans)-6-(1,3-benzodioxol-5-yl)-2,3,6,7,12,12a-hexahydro-2methyl-pyrazino[1′,2′:1,6]pyrido[3,4-b]indole-1,4-dione, has the formula C22H19N3O4 representing a molecular weight of 389.41 g/mol and is depicted by the following chemical structure:

Tadalafil was originally developed by ICOS in 1994 and marketed via collaboration with Eli Lilly that subsequently took over ICOS in 2007. Tadalafil is a white solid particulate material that is practically insoluble in water. It is sparingly soluble in tetrahydrofuran and chloroform but is soluble in dioxane. Tadalafil has an apparent partition coefficient (log P)app=1.7 in n-octanol/buffer pH 7.4.
Tadalafil, currently used for the treatment of erectile dysfunction (ED) under the Cialis® brand name, is a cGMP (cyclic gaunosine monophosphate) specific phosphodiesterase 5 (PDE5) inhibitor which enhances the effect of nitric oxide at the nerve ending and endothelial cells in the corpus cavernosum by inhibiting phosphodiesterase type 5 in the corpus cavernosum of the penis. This results in vasodilation, increased inflow of blood into the corporus cavernosum, and ensuing penile erection upon sexual stimulation. Tadalafil is also being investigated currently as a treatment for pulmonary hypertension. For ED treatment tadalafil is manufactured as a tablet for daily use (2.5 and 5 mg) with doses ranging from 5-20 mg as needed. See Cialis® Prescribing Information, February 2010. Tadalafil, the method of preparing tadalafil, its therapeutic uses, and pharmaceutical compositions containing it are described in U.S. Pat. Nos. 5,859,006; 6,140,329; 6,821,975; 6,943,166; and 7,182,958 which are incorporated herein by reference in their entirety.
A major limitation on its use to treat ED, which could be due in part to its very low aqueous solubility, is the fact that tadalafil takes from 0.5 to 6 hours (Tmax) to achieve the mean peak plasma concentration (Cmax) regardless of food intake. Therefore enhancing the aqueous solubility and subsequently the rate of dissolution can lead to a shortened Tmax and produce a quicker onset of action. In addition, this reduces the residence time in the GI tract and decreases the possibility of producing undesirable side effects through long contact with the GI membrane.
Different crystalline polymorphic forms of tadalafil have been disclosed in the literature (see for example U.S. Patent application 2006/0111571 A1). There are eight known polymorphs including forms II and III which are acetone and methylethyl ketone solvates, respectively.
In general, active pharmaceutical ingredients (APIs) in pharmaceutical compositions can be prepared in a variety of different chemical forms including chemical derivatives, solvates, hydrates, cocrystals and/or salts. Such compounds can also be prepared in different physical forms of the same chemical composition. For example, they may be amorphous, may have different crystalline forms (polymorphs), and/or may exist in different solvated or hydrated states with no change in the chemical composition of the original API. The discovery of new crystalline forms of a pharmaceutically useful compound may provide an opportunity to improve the physico-chemical characteristics and subsequently the clinical performance of such a compound. Additionally it expands the array of resources available for designing, for example, a pharmaceutical dosage form of an API with a targeted/sustained release profile or improved shelf life due to higher physical stability, improved particle size or size distribution, or powder flowability and handling for easier downstream processing or even for changing its route of delivery.
For example, crystalline forms of the same chemical composition (polymorphs) can have different aqueous solubilities from one another, where typically the more thermodynamically stable the polymorph the less soluble it becomes. In addition, polymorphs of an API can also differ in physico-chemical properties such as solid phase stability, higher shelf-life, bioavailability, particle morphology, vapor pressure, density, color, melting point and compressibility. However, it is sometimes difficult to produce the desired polymorph consistently or there is little improvement in their aqueous solubility or characterizing of all polymorphic forms can be challenging. In addition, if only one polymorph is discovered then there is less opportunity to manipulate and improve physico-chemical properties of the API. These limitations can adversely affect the API formulation into a usable dosage form or even force pharmaceutical companies to abandon the API's development.
Therefore it would be highly appreciable to generate novel crystalline molecular complexes, such as cocrystals, that can address such limitations and enhance the properties of an API such as aqueous solubility, rate of dissolution, bioavailability, Cmax, Tmax, solid phase stability, shelf life, downstream processibility (e.g. flowability, compressibility, degree of brittleness, particle size and size distribution), crystallization of amorphous compounds, decrease polymorphic form diversity, reduce toxicity, taste masking, and/or its production and manufacturing methods efficiency. For oral delivery of solid dosage forms, it is frequently advantageous to have novel crystalline forms of drug materials that possess such improved properties and particularly increased aqueous solubility and solid phase stability. It is also desirable in general to increase the dissolution rate of such solid forms, increase bioavailability, and provide a more rapid onset to quicken the therapeutic effect. In addition, it is useful to have a crystal form which, when administered to a subject, reaches a peak plasma level faster and has a longer lasting therapeutic plasma concentration, when compared to other existing forms on a dose-for-dose basis.
Because of the limitations related to the low aqueous solubility of tadalafil, there is a need to develop novel forms that have improved physico-chemical properties including aqueous solubility, which can be formulated for use in oral administration to achieve faster onset or possibly improve its clinical profile.
It is possible to tackle these limitations by generating novel crystalline forms of tadalafil that includes crystalline molecular complexes, such as cocrystals, thereof that can improve the rate of dissolution, shorten Tmax, achieve rapid onset and reduce residence time in the GI tract.